1. Field of the Invention
A present invention relates to a wireless communication device and a wireless communication method in an MIMO transmission system using linear precoding.
2. Related Background Art
High-speed, highly reliable MIMO-OFDM transmission systems are attracting attention in mobile communications. OFDM signals are composed of a large number of subcarriers, and although they have properties suitable for frequency-selective phasing, they have a problem of a large peak-to-average power ratio (PAPR). In addition, in the case of carrying out code multiplexing and transmitting a plurality of signal series after spreading in a CDMA system as well, there is a problem of the PAPR becoming excessively large. Moreover, in addition to OFDM and CDMA systems, in the case of using a single carrier system and the like, in the case of carrying out MIMO transmission using a plurality of transmission antennas, there is a problem of the PAPR becoming excessively large when the number of signal streams to be parallel multiplexed increases.
In this manner, in the case of transmission signal waveforms having a large PAPR, together with transmission characteristics deteriorating due to occurrence of nonlinear distortion in a transmission power amplifier, there is also occurrence of out-band emission power. When a larger input backoff is used for the transmission power amplifier in order to avoid this, transmission power efficiency decreases considerably. Consequently, subcarrier phase hopping-selected mapping (SPH-SLM) that is capable of realizing both improvement of transmission characteristics in MIMO-OFDM and reduction of PAPR is described in a document (S. Suyama, N. Nomura, H. Suzuki, and K. Fukawa, “Subcarrier phase hopping MIMO-OFDM transmission employing enhanced selected mapping for PAPR reduction,” Annual IEEE Inter. Symposium on Personal Indoor Mobile Radio Communication. (PIMRC), pp. 1-5, September 2006; hereinafter called “Document 1”). In the method described in the Document 1, PAPR is able to be reduced by selecting a phase pattern that results in maximum peak suppression from random phase matrix candidates in the form of a plurality of unitary matrices that cause phase rotation of a modulation signal of each subcarrier.
However, in the case of applying this method for multiplying a random phase matrix of the related background art described above to an MIMO transmission system using linear preceding such as MIMO eigenmode transmission, there was a problem of being unable to obtain effects of linear precoding. In other words, there was a problem of the characteristics of beams formed by multiplying transmission antenna weight by each transmission signal stream in the form of linear precoding processing being deteriorated by applying the related background art described above. For example, in a case of using MIMO eigenmode transmission for linear precoding processing, although a plurality of formed beams have a characteristic of being orthogonal, in a case of applying the above-mentioned related background art in the form of a method for multiplying a random phase matrix to this MIMO eigenmode transmission, the formed orthogonal beams end up deteriorating.